why does javascript return -0

Question

I am trying to write a code that shows the last digit (in a set of 3) is the same. I could not understand why I got false for each set so, I decided to test -20%10 and I got -0 meanwhile 20%10 gives me the expected 0. What is this -0 about?

function last_digit( x, y, z){

if(x % 10 == y % 10 == z % 10 ){
return true;

}else {
return false;

}

}
console.log(last_digit(20, 30, 400));
console.log(last_digit(-20, 30, 400));
console.log(last_digit(20, -30, 400));
console.log(last_digit(20, 30, -400));

Your comparison is equal to x % 10 == (y % 10 == z % 10). I.e. you compare x % 10 with a boolean result. What you want is something like x % 10 == y % 10 && y % 10 == z % 10. — Some programmer dude
– Some programmer dude, Commented Oct 9, 2017 at 13:58
@appletree, please use === for (pretty much) all comparisons. why? because true == 1 check last_digit(5, 15, 21) with your code. — Thomas
– Thomas, Commented Oct 9, 2017 at 15:31
I think in playing around with the code to try to understand why I was getting -0, my if clause got botched. I didnt realize I posted it this way, and I am grateful for the help. Cheers all! — appletree
– appletree, Commented Oct 10, 2017 at 5:00

Nina Scholz · Accepted Answer · 2017-10-09 13:58:40Z

1

You need to compare the values pair wise with a logical AND &&, because the first comparison returns a boolean value and the next is comparing this value with the rest of the following expression.

function last_digit(x, y, z) {
    return x % 10 == y % 10 && x % 10 == z % 10;
}

console.log(last_digit(20, 30, 400));
console.log(last_digit(-20, 30, 400));
console.log(last_digit(20, -30, 400));
console.log(last_digit(20, 30, -400));

answered Oct 9, 2017 at 13:58

Nina Scholz

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1 Comment

appletree Over a year ago

No wonder, I was not getting true for all of them. Thanks Nina

ofey · Accepted Answer · 2017-10-09 14:07:13Z

1

Do it this way,

function last_digit(x, y, z) {

  if ((x % 10) == (y % 10) && (y % 10) == (z % 10)) {
    return true;

  } else {
    return false;

  }

}
console.log(last_digit(20, 30, 400));
console.log(last_digit(-20, 30, 400));
console.log(last_digit(20, -30, 400));
console.log(last_digit(20, 30, -400));

answered Oct 9, 2017 at 14:07

ofey

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